1. Field of the Invention
The present invention relates to a composition for film formation, which can form a porous film which excels in dielectric properties, adhesion, film uniformity and mechanical strength and has reduced moisture absorption; a porous film and a method for forming the same; and a semiconductor device which contains the porous film inside.
2. Description of the Related Art
In the fabrication of semiconductor integrated circuits, as the circuits are packed tighter, an increase in interconnection capacitance, which is a parasitic capacitance between metal interconnections, leads to an increase in interconnection delay time, thereby hindering the enhancement of the performance of semiconductor circuits. The interconnection delay time is called an RC delay, which is in proportion to the product of the electric resistance of the metal interconnections and the static capacitance between the interconnections. Reducing the interconnection delay time requires reducing the resistance of metal interconnections or the interconnection capacitance.
One of the methods for reducing the interconnection capacitance includes lowering the dielectric constant of the interlevel insulator film between the metal interconnection. Then, a porous insulator film has been proposed. Since pores in the film can decrease mechanical strength, however, an insulating film having both a low dielectric constant and high intensity has been demanded.
A first method for forming a porous film is as follows: a precursor solution of a siloxane polymer containing a thermally unstable organic component is synthesized; then the precursor solution is applied on the substrate to form a coating film; and later, a heat treatment is applied to decompose and volatilize the organic component. The result is a number of micro-pores formed in the film.
As one of the methods for forming an insulator film includes a method comprising steps of preparing a siloxane polymer precursor containing a thermally unstable organic component, applying a solution of the precursor to a substrate to yield a film, and heating the film so as to decompose and evaporate the organic component so that the space the organic component has occupied becomes pores. However, there is a problem that a volatile component and decomposed substance remain and cause the electrical property degradation and the interface separation of the insulator film during a semiconductor device production process.
Moreover, a composition comprising low to medium molecular weight siloxane polymer is proposed (Japanese Patent Provisional Publication No. 2001-164186). Herein, a high molecular weight siloxane polymer is produced by hydrolysis and condensation of a silane compound in the presence of alkaline catalyst and is subjected to hydrolysis and condensation the presence of acidic catalyst together with a silane compound so as to form the low to medium molecular weight siloxane polymer. However, dielectric constant of the composition is not sufficiently low. Also proposed is a high molecular weight siloxane polymer which is obtained through hydrolysis and condensation of a silane compound, in the presence of the organic base or inorganic salt radical. However, the film strength of the high molecular weight siloxane polymer is insufficient.
As mentioned above, the conventional materials have problems such as deterioration of film quality during a heat treatment step and higher cost. In addition, enlargement of pore diameters take place during the formation of porous film, making it difficult to obtain the low dielectric constant. Further, when the conventional porous film is incorporated into the multi-level interconnects of the semiconductor device as an insulator film, the mechanical strength necessary for the semiconductor device is not obtained.
Thus, when the dielectric constant of the porous film used as an insulator film in the multi-level interconnects of the semiconductor device is too high, the RC delay in the multi-level interconnects of the semiconductor device is increased. Consequently, the performance of the semiconductor device (high speed and low power consumption) has not been improved. Furthermore, a porous film having a low mechanical strength lowers the reliability of the semiconductor device.